Minimally Invasive Surgical (MIS) procedures aim to minimize damage to healthy tissue by accessing targeted organs and anatomical cavities through relatively small size incisions. Since the workspace is not fully exposed, the surgeon typically carries the medical procedure using as guidance video acquired by a camera system that is inserted into the cavity. MIS procedures are being increasingly adopted in different medical specialties, such as orthopedics, abdominal surgery, urology, neurosurgery, and ENT, just to name a few.
Arthroscopy is a MIS procedure for treatment of damaged joints in which instruments and endoscopic camera (the arthroscope) are inserted into the articular cavity through small incisions (the surgical ports). Arthroscopy, as opposed to conventional open surgery, largely preserves the integrity of the articulation, which is beneficial for the patient in terms of reduction of trauma, risk of infection and recovery time. Unfortunately, arthroscopic procedures are relatively difficult to execute because of indirect visualization and limited maneuverability inside the joint, with novices having to undergo a long training period and experts often making mistakes of clinical consequences. This is a scenario where computer-assistive technologies for safely guiding the surgeon throughout the procedure can make a difference, both in terms of improving clinical outcome and in terms of decreasing the surgeon learning curve.
Depending on the particular clinical application, a system for Computer-Aided Surgery (CAS) comprises two distinct stages: (i) an offline step in which the procedure is planned leading to some sort of computational model that can either be a three-dimensional (3D) pre-operative image of the patient's organ (e.g. CT-Scan), a statistical bone model, or a set of guidelines for inferring meaningful locations with respect to anatomical landmarks; and (ii) an intra-operative navigation step in which the computer guides the surgeon throughout the procedure for the execution to be done as defined.
The intra-operative navigation usually passes by overlying the pre-operative computational model with the actual bone, and by localizing in real-time the tools and instruments with respect to each other, and with respect to the targeted organ. Typically, the technology to accomplish this task is Optical-Tracking (OT) that consists in using a stationary stereo head, henceforth called base station, for tracking a set of markers that are rigidly attached to instruments and/or bone. The stereo head comprises two infrared (IR) cameras that track a set of point markers that are rigidly attached to the object of interest. The position of each marker is estimated by simple triangulation and, since their relative arrangement is known ‘a priori’, the 3D pose of the object of interest is computed in the reference frame of the base station. Recently, a technological variant of OT was introduced in which the two IR cameras are replaced by two conventional video cameras operating in the visible spectrum, and the arrangements of IR markers are replaced by planar markers with printed known patterns.
The surgical navigation solutions that are currently available for Orthopedics, Neurosurgery, and ENT invariably rely in OT. In generic terms, the typical workflow passes by the surgeon to rigidly attach a tool marker to patient and/or targeted organ, which is followed by pin pointing anatomical landmarks with a calibrated tracked probe. The 3D position of these landmarks is determined in the coordinate system of the base station and the pre-operative computational model is registered with the patient. From this point on, it is possible to determine in real-time the pose of instruments with respect to patient and plan, which enables the system to safely guide the surgeon throughout the procedure. There are some variants to this scheme that mainly address the difficulties in performing the 3D registration of patient's anatomy with a pre-operative model with a tracked probe that tends to be an error prone, time consuming process. For example, the O-arm from Medtronic® combines OT with a CT-scanner that enables the acquiring of the 3D pre-operative model of patient's anatomy in the Operating Room (OR) before starting the procedure, which avoids the surgeon performing explicit registration. The system that is being developed by 7D Surgical® goes in the same direction with the 3D model being obtained using multi-view reconstruction and structured light to avoid the ionizing radiation of CT-scanning. Nevertheless, these systems still rely in conventional OT to know the relative position between instruments and anatomy after registration has been accomplished.
OT has proved to be an effective way of obtaining real-time 3D information in the OR, which largely explains the fact of being transversally used across different systems and solutions. However, the technology has several drawbacks that preclude a broader dissemination of surgical navigation: (i) it requires a significant investment in capital equipment, namely in acquiring the base station; (ii) it disrupts normal surgical workflow by changing the OR layout to accommodate additional equipment, by forcing the surgeon to work with instruments with bulky tool markers attached, and by constraining the team movements due to the need of preserving lines of sight between base station and tool markers; and (iii) it is not well suited to be used in MIS procedures because organs and tissues are occluded which avoids placing marker tools that can be observed from the outside by the base station. For example, OT based navigation in arthroscopic procedures always requires opening additional incisions such that the marker tool attached to the bone protrudes through patient skin.
In recent years some alternative technologies have emerged in an attempt of obviating the above-mentioned drawbacks. Electromagnetic Tracking (ET) is currently used in some surgical navigation systems with the advantage of not requiring preservation of a line of sight. However, it has the problem of being vulnerable to electromagnetic interference caused by nearby metals and devices, being in practice less reliable and accurate than OT. Moreover, it still requires additional capital equipment, namely a base station, and the need of attaching coil markers with hanging wires to organs makes it non amenable to MIS procedures.